Above we have two spoke routers and one hub, our underlay network uses 192.168.123.0/24 and the overlay network uses 172.16.123.0/24. Each router has a loopback interface with a network that we can advertise in RIP.

Our hub router has learned both networks on the loopback interfaces. Take a good look at the next hop IP addresses that we have here, these are the tunnel addresses. When the hub wants to reach these networks, it will have to check NHRP to find the NBMA addresses:

The NBMA in this particular example refers to the nature of the actual physical network, and the addressing of the physical interfaces connected to the medium. That is, the 192.168.123.X subnet. So if this is what you mean by the whole underlay network, then yes you are correct.

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